Display devices such as cathode ray tubes and liquid crystal displays are well known image display devices as are their characteristics. Cathode ray tubes, for example, are known to have good characteristics with respect to color, brightness, contrast and resolution but are bulky and consume relatively large amounts of power. In contrast liquid crystal displays are relatively compact and provide flat panel displays useful in lap top computers and the like. However, such liquid crystal devices provide relatively poor contrast in comparison with cathode ray tube displays and provide only a limited angular display range. Moreover, color liquid crystal display devices consume power at rates incompatible with extended battery operation.
More recently, thin film field emission displays (FEDs) have become increasingly important for applications requiring light weight portable screens with good display characteristics. Passive FED devices in the elementary form as illustrated in FIG. 1 conventionally include a substrate 11 onto which a resistive material layer 12 is deposited for providing current limit resistor for field emission tip 13. Surrounding the emitter tip 13 is an extraction gate or grid structure 15 which is supported by insulating layer 14. A screen comprising a glass layer 16, a transparent conductive layer 17 and phosphors 18 function as the high voltage anode. When the voltage source provides the illustrated relative potential differences, an electron stream is emitted by the emitter 13 toward the phosphor coated screen so as to provide the illustrated luminescent display.
In more detail FIG. 2 illustrates a cross-sectional view of a portion of a flat panel field emission display wherein a single display segment 2 is shown. Each such display segment is capable of displaying a pixel of information or a portion of a pixel. In this regard a pixel may include one or a plurality of emitters depending on the size and type of display. For example, one or more emitters may be applied to each of a red-green-blue full-color triad pixel. The elements depicted in FIG. 2 generally cooperate in the manner noted above with regard to the structure of FIG. 1 and the same numerical labeling has been applied to corresponding elements found in both FIGS. 1 and 2. However, since proper functioning of the emitter tips requires operation in a vacuum, FIG. 2 further shows the use of spacers 19 for supporting the screen 16 over the base assembly against atmospheric pressure. Moreover, the cathode base electrodes 12 are illustrated as being patterned for the purpose of obtaining selectable activation of a display segment or pixel such that the controller 24 can establish a voltage differential between the selected emitter tips and the anode structures through the use of a matrix of pixels that are addressable via column and row control signals.
Various techniques and drive circuits are known for selectable activation of the emitters associated with a pixel. For example, the base electrode conductors 12 could be arranged in rows and the grid 15 arranged in columns perpendicular to the rows of cathode base electrodes. Controller 24 would apply appropriate row address signals to the cathode base electrodes and column control address signals to the grid column segments connected to appropriate voltage potentials to selectively activate the emitters of the desired pixel. Suitable pixelator drive circuitry for the rows and columns is known in the art and is disclosed, for example, in commonly owned U.S. Pat. No. 5,438,240 issued Aug. 1, 1995 to Cathey et al and U.S. Pat. No. 5,410,218 issued Apr. 25, 1995 to Hush which are hereby incorporated by reference in their entirety.
Alternatively, the base electrode conductors 12 may be patterned so as to be arranged as a matrix which is addressable via column and row control signals to selectively switch appropriate voltages so as to activate the emitter tips of a selected pixel. Driving circuits for use with the alternative arrangement are disclosed, for example, in commonly owned U.S. Pat. No. 5,357,172 issued Oct. 18, 1994 to Lee et al, U.S. Pat. No. 5,387,844 issued Feb. 7, 1995 to Browning and U.S. Pat. No. 5,459,480 issued Oct. 17,1995 to Browning et al. These patents are also hereby incorporated by reference in their entirety.
Developments in thin film field emission display technology have provided inexpensive low power devices with full color, high resolution and high contrast capabilities. With regard to reliability it is important to monitor operating parameters for the purpose of detecting failures such as inoperable emitter tips as well as investigating the causes thereof both before and after the display devices are vacuum packaged.
For detecting failures and for diagnostic purposes, it is important to be able to determine single pixel anode currents at a reasonably high speed and with good current measurement resolution.